wallet: Update best block record after block dis/connect

When a block is connected, if the new block had anything relevant to the
wallet, update the best block record on disk. If not, also sync the best
block record to disk every 144 blocks.

Also reuse the new WriteBestBlock method in BackupWallet.

test/functional/wallet_reorgsrestore.py

@@ -119,11 +119,11 @@ class ReorgsRestoreTest(BitcoinTestFramework):
         self.start_node(0)
         assert_equal(node.getbestblockhash(), tip)
 
-        # Due to an existing bug, the wallet incorrectly keeps the transaction in an abandoned state, even though that's
-        # no longer the case (after the unclean shutdown, the node's chain returned to the pre-invalidation tip).
-        # This issue blocks any future spending and results in an incorrect balance display.
+        # After disconnecting the block, the wallet should record the new best block.
+        # Upon reload after the crash, since the chainstate was not flushed, the tip contains the previously abandoned
+        # coinbase. This should be rescanned and now un-abandoned.
         wallet = node.get_wallet_rpc("reorg_crash")
-        assert_equal(wallet.getwalletinfo()['immature_balance'], 0) # FIXME: #31824.
+        assert_equal(wallet.gettransaction(coinbase_tx_id)['details'][0]['abandoned'], False)
 
         # Previously, a bug caused the node to crash if two block disconnection events occurred consecutively.
         # Ensure this is no longer the case by simulating a new reorg.